Electrically conductive polymer composites often consist of electrically insulating polymers filled with electrically conductive fillers. Such fillers often consist of metal- or carbon-based fillers often in the form of flake or fibers. In order to make the composite conductive, the fillers are added to the point a critical filler concentration is reached at which the composite changes from electrically insulating to electrically conducting. This concentration, termed the percolation threshold, is associated with a continuous electrical pathway formed from the touching or “percolation” of conductive filler particles. Beyond this threshold, the electrical conductivity can be further improved by addition of filler to the polymer matrix. The ultimate conductivity beyond the threshold will depend on the type of filler used and the maximum obtainable filler loading before tradeoffs in other composite properties becomes unacceptable from an application standpoint
Electrically conductive polymer composites are commonly used for such applications requiring electromagnetic shielding, electrostatic discharge, or high conductivity for device interconnects or circuitry. The type of application will dictate the ultimate conductivity needed which will dictate the type and concentration of conductive fillers used in the composite material. In many instances the level of filler required leads to undesirable sacrifices in other important physical characteristics of the composite, such as dispense viscosity, adhesion, impact strength, among other things. In some instances, the cost of the filler is a limiting factor, particular for such fillers as gold, silver, or carbon fibers. It would be thus desirable to achieve high levels of electrically conductivity with minimal loss in polymer attributes.
In the area of thermally conductive applications, surface mounting of electronic composites via interface adhesives is well developed in automated package assembly systems. Such adhesives are used in several approaches to provide lid attach, sink attach and mainly thermal transfer from flip chip devices, as well as against mechanical shock and vibration encountered in shipping and use. As semiconductor devices operate at higher speeds and at lower line widths, the thermal transfer properties of the adhesive are critical to device operation. The thermal interface adhesive must create an efficient thermal pathway between the die or lid and the heat sink as the adhesive itself due to interface resistance and bulk resistance is typically the most thermally resistant material in the die-adhesive-lid-adhesive-sink or die-adhesive-sink configuration. The thermal interface adhesive must also maintain efficient thermal transfer properties through reliability testing which simulates actual use conditions over the life of the device. Moreover, a suitable adhesive must have certain fluid handling characteristics, and exhibit specific adhesion, controlled shrinkage, and low corrosivity in order to provide long term defect-free service over the thermal operating range of the electronic circuit package.
As with traditional electrical applications mentioned above, interface adhesives having high bulk thermal conductivities are often made by adding large levels of thermally conductive filler to the reactive organic resin. In many instances, undesirable increases in viscosity occur to the point handling (or dispensing) becomes an issue which often limits the thermal conductivity that can be achieved. To help overcome this issue, low molecular species, such as non-reactive solvent, plasticizer or other liquid viscosity reducing diluents are added to the formulation. However, a downside to this approach, as seen in epoxy based formulations, is these low molecular weight species cause shrinkage issues, void formation, and delamination when the adhesive is cured.
Other approaches for obtaining high bulk thermal conductivities have employed fillers that are known to sinter as temperatures amenable to electrical devices processing temperatures. For example, U.S. Pat. No. 7,083,850 entitled “Electrically Conductive Thermal Interface” describes a porous, flexible, resilient heat transfer material which comprises a network of metal flakes. The material is made by forming a conductive paste comprising a volatile organic solvent and conductive metal flakes. The conductive paste is heated to a temperature below the melting point of the metal flakes, thereby evaporating the solvent and sintering the flakes only at their edges. While highly thermally conductive, this material is quite limited in its ability to adhere to surfaces and has an intrinsically high modulus owing to pure filler remaining once the solvent is removed. Moreover, most packaging processes prefer low solver to solvent-less materials owing to the complexities and environment concerns with removing solvent.
It would thus be desirable to sinter metal flakes within interface material obtaining adhesive. Unfortunately, sintering in not achieved at low to moderate fillers loadings. This limitation is associated with the lack of direct filler particle contacts required for filler to occur due to the matrix material that coats them. It is only at very high volume percent filler that some sintering occurs, but at such concentrations the unreacted adhesive composition becomes extremely viscous and even solid-like and lacks the desirable polymeric attributes such as good adhesion, toughness, etc. It is this reason that existing approaches have resorted to using considerable amounts of solvents to address the viscosity issue which again has its downsides as mentioned earlier.
To this end, it would be desirable to provide a solvent-free (or low solvent) adhesive composition comprising a matrix polymer and low to moderate levels of filler material which exhibits high conductivity resulting from sintered filler, and also provides adhesive properties while maintaining beneficial properties such as dispensability, proper coefficient of thermal expansion, improved toughness, shock and vibration resistance, environmental protection, and the like.
It is further desirable to provide a homogeneous material in the unreacted state comprising filler particles in a reactive organic matrix whose properties and cure condition could be controlled to generate a heterogeneous structure during curing and whose final properties exhibit high levels of thermal and/or electrical conductivity among other attributes.